Electrostatic printing element comprising a dye sensitized zinc oxide coating on a high resistivity paper backing sheet and a process of using said element



United States Patent Oflice 3,476,559 Patented Nov. 4, 1969 3,476,559ELECTROSTATIC PRINTING ELEMENT COMPRIS- ING A DYE SENSITIZED ZINC OXIDECOATING ON A HIGH RESISTIVITY PAPER BACKING SHEET AND A PROCESS OF USINGSAID ELEMENT Irving Panken, Brooklyn, N.Y., assignor, by mesneassignments, to lug. C. Olivetti & C. S.p.A., Ivrea, Italy, acorporation of Italy No Drawing. Continuation-f application Ser. No.379,724, July 1, 1964. This application Jan. 29, 1968, Ser. No. 701,468

Int. Cl. G03g 5/08 U.S. Cl. 961.7 7 Claims ABSTRACT OF THE DISCLOSURE Anelectrostatic recording element is provided having a paper base sheet,zinc oxide as a photoconductor, a combination of sensitizing dyes forthe photoconductor, and a binder for the dye and photoconductor forbinding the two on the sheet. The recording element comprising the abovecombination of elements is panchromatically sensitive and possesses aresistivity, attributable to the base sheet only, from 10 ohm-cm. to 5l0 ohm-cm. Moreover, this resistivity is displayed at low humidityconditions of 15% relative humidity at 70 F. while at high humidityconditions, e.g., 90% at 70 F., the recording element is stillsuccessfully utilized. Of the dyes used, Thioflavin is preferred in anamount of 35% of the other dyes used, such as Auramine O, BromophenolBlue, and Rose Bengal.

This application is a continuation of application Ser. No. 379,724,filed July 1, 1964 and now abandoned.

This invention relates to electrostatic recording elements, a process ofelectrostatic recording, and, in particular, to an improvedpanchromatically sensitive paper and an improved photoconductive coatingfor such paper.

In operation a typical electrostatic printing process utilizes anelement, such as a paper sheet, coated with a composition comprising aphotoconductive material bonded by a resin or held in a matrix. Such acoating, in the dark, has a high resistivity. When the photoconductivecoating is exposed to light, such as that reflected from a page to becopied, the photoconductive material, on those parts of the coatingwhich are exposed to the light, becomes conductive compared to thematerial which is not exposed to light or only exposed to a relativelysmall amount of light. An electrostatic charge is imparted to thesurface of the element by means known to the art, for example bycorona-discharge, and this charge is dissipated where thephotoconductive material on the surface of the sheet becomes conductiveby exposure to light. Portions of the surface which have not beensubjected to any significant amount of light remain in a chargedcondition, thereby forming an invisible pattern or-latent image ofelectrostatic charges on the element. This pattern, for example,corresponds to the dark areas, e.g., the printing on the page to becopied.

The electrostatic image pattern is made visible by applying to theelement a toner powder which is selectively attracted and held by theelectrostatical charges of the pattern. This toner powder is convertedto a permanent printing by heating sufficiently to fuse the resin orother powder in the toner.

Apparatus for applying static charges to the photoconductive surface,exposing the surface to the desired light pattern, distributing, andfusing the toner on the surface are well known in the art.

The use of dye-sensitised photoconductive surfaces for obtainingpanchromatically-responsive surfaces for use in electrostatic printingprocesses is old in the art. Greig describes the use of dyes in thepreparation of panchromatic zinc oxide-organic binder coatingcomposition in U.S. Patent 3,052,540. It has been generally acceptedthat such light-sensitive photoconductive compounds must be used on ahigh resistance conductive base, i.e., one having a resistivity of up toabout 10 or 10 ohms-cm. under conditions of normal humidity andtemperature, to form electrostatic recording elements. Ordinary paperhad been considered too non-conductive to be useful as a carrierelement. However, the resistivity of the paper decreases with increasedmoisture content but at higher humidity it increases. Either excessivelyhigh or low conductivity of the carrier sheet may make impossible theimparting of a suitable static charge and maintaining the latent imagefor a suitable period of time.

To avoid excessively high resistivity during periods of low humidity,hurnectants are added to such paper; in some cases, water-receptiveresins are coated on the backside of the paper. Such treated papers,however, have inferior performance characteristics during periods ofhigh humidity because of excessive conductivity resulting from increasedmoisture content of the paper.

Thus it is a problem in this art to provide a backing that will besuitable at any normal temperature and humidity conditions. Among otherproblems that are common to the art is the obtaining of sharp contrastbetween the area in which the toner is fused and the light backgroundareas of the photoconductive surface, also the minimizing of timerequired for imparting a desired conductivity to the light areas of thephotoconductive surface on its being exposed, and maximizing the timethe electrostatic image will remain on the paper after initially beingimparted thereto.

Another problem in the preparation of suitable photoconductive surfaces,e.g., for use in oflice copying, etc., is the difiiculty in providing apanchromatically photoconductive surface which is substantially white,i.e., free from tints imparted by different dyes utilized to obtain thedesirable panchromatic response to light irradiation.

Therefore, it is an object of this invention to provide a copying sheetcoated with a photoconductive material having a substantially whiteappearance and preferably a panchromatic response to light irradiation.

It is another object of this invention to provide a conductive sheetwhich requires only a relatively short period of exposure to light inorder to discharge the surfaces exposed to such light.

Itis likewise an object of the invention to provide a composition ofrapid photoconductive response.

It is another object of the invention to provide a photoconductive paperof sufficiently high resistance to facilitate the acquisition andretension, in the dark, of a relatively high surface charge on thephotoconductive surface thereof when subjected to static-producing meanssuch as corona.

It is still another object of the invention to provide a photoconductivesheet, with a base material useful for substantially trouble-freecopying over a wide range of humidity conditions.

Applicants have met these objects by utilizing a paper backing having arelatively high resistivity as measured through the backing at 15relative humidity and 70 F. Use of such a sheet of high resistivity,i.e. of 10 ohmcm. up to about 5X10 ohm-cm, has been found to givesuccessful operation of electrostatic printing processes even underconditions of very high humidity, obtaining and maintaining highelectrostatic charges on the photoconductive surface of the backing, andpermitting the photoconductive surface to have suitable electrostaticdark-decay factor of about 0.1, that is the voltage across thephotoconductive surface will decay from 200 volts to 100 volts in about10 seconds. Such high-resistivity backings may be utilized successfullyin electrostatic printing processes even after being stored for extendedperiods at relative humidities as high as 90% at 70 F.

The photoconductive surface of the sheet responds to light withexcellent and surprising rapidity. This rapidity is thought to be due tothe sensitizing dyes utilized by applicants. These sensitizing dyes alsohave the advantage of contributing relatively little reddish oryellowish tint to the paper.

As a photoconductive coating useful with this high resistance paperbacking, there may be used a composition generally comprising from 10%to 40% of an organic polymer binder, 65 to 90% of photoconductive zincoxide, and from to about 0.5% of photosensitizing dyes.

Especially advantageous photoconductive coating materials are thosecomprising photoconductive agents such as zinc oxide (especially thatsold by New Jersey Zinc Company under the trade name Florence Green-Seal8) and resin binders such as those known to the art as petroleumhydrocarbon type resin e.g., as sold by Pennsylvania Industrial ChemicalCorporation under the trade name Piccopale 100, which is soluble inhydrocarbons, most esters and ketones, in some alcohols; copolymers ofstyrene and butadiene, e.g., that sold under the trade designationPliolite S-D by Goodyear Tire and Rubber Company, acrylic resins, e.g.,those sold under the trade designations B-82, B-44, and B72 by Rohm &Haas, terpolymers such as the indenstyrene-acrylonitrile terpolymer,sold under the name Piccoflex 105, by the Pennsylvania IndustrialChemical Corporation, and other resins known to the art to providesuitable nonconductive coatings which can carry and bind thephotoconductive agents dispersed therein for coating on a backing.

Plasticizers are useful on such a coating for modifying the film-formingproperties of these resins, advantageously the vinyl-type resinplasticizer sold under the trade designation Hercoflex 150 by HerculesPowder Company; but other plasticizers known in the art to be useful canserve this purpose, being chosen to suit the particular resin and tokeep the low-conductivity of the coating.

Dyestuffs which may advantageously be added, to extend the range ofchromatic response of the photoconductive coatings, include among othersthose listed in U.S. Patent 3,052,540. An advantageous fast-actingcomposition particularly adapted for use on the high-resistivity sheetof the present invention comprises Thioflavin, Auramine O, BromophenolBlue, and Rose Bengal with proportions adjusted to give a balancedspectral absorption. Most advantageous dyestuff mixtures comprise atleast 35% or more of Thioflavin. Some of these dyes may be omitted orreplaced by equivalent dyes.

A toner, i.e. a thermally activatable adhesive powder (with or withoutcolor) adapted to contrast with the coating, is distributed over thecoated sheet and is held electrostatically on the charged areas whichhave not been rendered conductive by light irradiation. Such toneradvantageously comprises a low conductivity carbon black, e.g. that soldunder the trade designation Raven by Columbian Carbon Company, and aresin binder essentially comprised of a heat activatable material suchas thermoplastic indene-styrene-acrylonitrile terpolymer sold under thetrade name Piccoflex 120 by Pennsylvania Industrial Chemical Corporationand a low molecular weight resin sold under the trade name PiccolasticP-lO O by the same company. These are mixed with a magnetic materialsuch as magnetic iron oxide powder to orient it into a magnetic brush.

Generally, the toner composition comprises from 1% to 25% carbonblack,5% to 40% iron oxide and 30% to 80% of resin binder.

The base sheet of high-resistivity may advantageously comprise aclay-coated paper, e.g., of sixty-pound Weight and thickness about0.0032 inch, obtainable from Allied Paper Company of Kalamazoo,Michigan. The photosensitive coating is applied at a rate of from 10 to.40 lbs. per ream (24" x 36" sheets), i.e., about 3,000 square feet ofpaper. An especially advantageous coat is about 21 lbs. per ream with acoating of about a 0.065 mil thickness.

In this application, I have described a preferred embodiment of myinvention and suggested various alternatives and modifications thereof;but it is to be understood that these are not intended to be exhaustive,and other changes and modifications can be made within the scope of theinvention. The particular descriptions and suggested variations hereinare selected for purposes of illustration in order that others skilledin the art will more readily and fully understand the invention andprinciples thereof and will be able to embody it in a variety of formsand to modify them, each as may be best suited to the conditions of aparticular use.

Example 1 The following materials were compounded by grinding,

e.g. in a paint mill, as used in the coating art until the zinc oxidewas dispersed to a fineness of 5-6 on the A dispersion of dyescomprising the following materials was then mixed into the composition:

Rose Bengal mg 10 Bromophenol Blue mg 2 Auramine O mg 10 Thioflavin P mg25 Methyl alcohol cc The resulting composition was coated on 60-poundbond paper having a resistivity of 2X10 cm. as measured at 15% relativehumidity and F, The coating was spread at a rate of about 0.007 lb. (dryweight) per square foot of paper and formed a coating of about 0.65 milsthick. This coating was dried. The coated paper formed an effectiveelectrostatic recording sheet.

The paper was stored under dark conditions and used in an electrostaticprinting machine, e.g. such as described in a copending application Ser.No. 295,652 of Ira Sage now known to the art, .and passed through a6,000-vo1t, corona-discharge unit to cause a polarization of electricalcharges on the coated paper and thus establish a negative charge on thecoated surface of the element. The element was then exposed to a lightimage, focused on the charged surface, giving light and shadowcorresponding to the image to be printed, e.g., a plurality of thinhorizontal black shadow lines. This light exposure lasted about 0.075second. After the exposure, the element was powdered with a tonercomprising 15 parts carbon black and 55 parts of a fusible resin bindermixed with 20 parts magnetic iron oxide; this mixture is then mixed withan iron powder, which powder acts as a magnetic brush when in a magneticfield. This binder was previously compounded to form an apparentlyhomogeneous composition and reduced to a powder having a particle-sizeof about 2 microns.

The sheet was inverted and shaken lightly to remove toner in unchargedareas; the toner, however, adhered to the areas or lines which had notbeen exposed to light. On heating this sheet, the retained toner wasfused to form a permanent printing resembling in appearance lines formedof India ink.

Example 2 The procedures used in Example 1 were repeated using thefollowing quantities of materials to form the photoconductive coatingcompositions:

Zinc oxide grams 210 Styrene-butadiene copolymer (Pliolite S-SD) grams29.5 Plasticizer (Hercoflex 150) do 6.5 Toluene do 1400 Rose Bengal mg18 Auramine O mg 21 Thioflavin P mg 40 Methyl alcohol cc 110 BromophenolBlue mg 4 What is claimed is:

1. A panchromatically-sensitive recording element for electrostaticcopying consisting essentially of .a paper backing sheet having aresistivity of from ohm-cm. to 5X10 ohm-cm. as measured at relativehumidity at 70 F., and a coating on said sheet comprisingphotoconductive zinc oxide and a sensitizing dye mixture dispersed in anorganic polymer-matrix, said sensitizing dye mixture comprising RoseBengal, Bromophenol Blue together with at least one dye selected fromthe group consisting of Thioflavin and Auramine O.

2. A panchromatically-sensitive recording element as in claim 1 whereinsaid sensitizing dye mixture comprises Rose Bengal, Bromophenol Blue,and Thioflaven.

3. A panchromatically-sensitive recording element as in claim 1 whereinsaid sensitizing dye mixture comprises Rose Bengal, Bromophenol Blue andAuramine O.

4. A panchromatically-sensitive recording element as in claim 1 whereinsaid sensitizing dye mixture comprises at least of T hioflavin.

5. A panchromatically-sensitive recording element as in claim 2 whereinsaid sensitizing dye mixture comprises a major amount of Thioflavin.

6. A process of electrostatic printing comprising imparting anelectrostatic charge to a recording element consisting of a paperbacking sheet having a resistivity of from 10 ohm-cm. to 5x10 ohm-cm. asmeasured at 15% relative humidity and F. and a photoconductive coatingon said sheet comprising a major portion of photoconductive zinc oxideand a sensitizing dye mixture dispersed in an organic polymer matrix,said sensitizing dye mixture comprising Rose Bengal, Bromophenol Bluetogether with at least one dye selected from the group consisting ofThioflavin and Auramine O, exposing said sheet to a light image, coatingsaid sheet with a toner pow der, removing excess toner to leave apattern on said element, and fusing said toner.

7. A process as in claim 6 wherein a major proportion of the dye mixtureis Thioflavin.

References Cited UNITED STATES PATENTS 3,052,540 9/1962 Greig 96-1.73,113,003 12/1963 Teiser et a1. 961.5 3,116,147 12/1963 Uber et a1.961.8 3,121,006 2/1964 Middleton et al. 96-15 OTHER REFERENCES Young eta1., R.C.A. Review, December 1954, pp. 469-478. Waltwin, TheLithographers Manual, vol. 2, Waltwin (1958) pp. 1333 to 13:33.

GEORGE F. LESMES, Primary Examiner I. C. COOPER, Assistant Examiner

